Multi-valorisation of giant reed (Arundo Donax L.) to give levulinic acid and valuable phenolic antioxidants

Up to now, on the industrial scale, the acid-catalysed hydrothermal conversion of lignocellulosic biomass has been targeted to the production of levulinic acid (LA), while the lignin fate has been neglected, and its use has been limited at most to the energy recovery. Now, an integrated investigation of the hydrothermal process for the synthesis of LA starting from giant reed (Arundo Donax L.) was studied taking into account the lignin phenol derivatives present in the liquid phase and in the solid hydrochar. The analysis of the hydrochar was carried out adopting coupled pyrolysis techniques, e.g. Py-GC/MS, TGA/FTIR, and EGA-MS, paying a special attention to the contribution of the evolved simple phenolics. The hydrochar washing fractions were analyzed for the total phenolic content (TPC) and antioxidant activity (AA), adopting TEAC, FRAP, and DPPH standard essays, comparing the antioxidant activities with those deriving from the starting LA-rich mother liquor. Two washing cycles allowed the complete recovery of the phenolic compounds of interest, leached from the porous hydrochar. Also the starting LA-rich liquor was of great interest for the same purpose, having the highest content of these compounds, which must be necessarily removed for the production of the commercial pure LA. Promising antioxidant properties were ascertained for the mother liquor and the hydrochar washings, obtaining good linear correlations between total phenolic compounds (TPC) and antioxidant capacity (TEAC, FRAP, and DPPH). This multi-valorisation approach contributes to improving the sustainability of the entire LA process and shifts the attention towards the antioxidants, new niche bioproducts of great interest, which add significant economic value to the overall Biorefinery of the lignocellulosic biomass.